1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser diode, and more particularly, it relates to a vertical cavity surface emitting laser diode used as a light source for optical information processing and optical communication and a light source for a data storage device storing data with light.
2. Description of the Related Art
A vertical cavity surface emitting laser diode (hereinafter, referred to as VCSEL) is a laser diode that emits light from a surface of a semiconductor substrate and has such characteristic features as (1) a low driving voltage, (2) capability of wafer level characteristic examination and (3) easiness in implementation, as compared to an edge emitting laser diode.
As shown in FIG. 26, a VCSEL has a substrate 2 having a lower electrode 1 formed thereon, and on the substrate 2, semiconductor thin films including a lower semiconductor multi-layer reflector 3, an active region 4, an AlAs layer 5 and an upper semiconductor multi-layer reflector 6 accumulated in this order. The uppermost layer of the upper semiconductor multi-layer reflector 6 is a contact layer 7, and an upper electrode 8 is disposed thereon through the contact layer 7. An aperture 9 in a circular shape determining an emission window of laser light is formed at the center of the upper electrode 8. A mesa or post 10 in a cylindrical shape is formed on the substrate 2 from the upper semiconductor multi-layer reflector 6 to a part of the lower semiconductor multi-layer reflector 3, and a bottom, a side wall and a part of a top end of the post 10 are covered with an interlayer dielectric film 11. An oxide region 12 is formed inside the post 10 by oxidizing a periphery of the AlAs layer 5 for obtaining a higher gain distribution.
Along with increase in data communication capacity in recent years, such a VCSEL is more strongly demanded that can be easily coupled with an optical fiber and is excellent in fast responsiveness. As a VCSEL that is improved in coupling efficiency with an optical fiber, particularly a plastic optical fiber, JP-A-2000-76682 proposes a VCSEL having a columnar semiconductor accumulation body having on an upper surface thereof a laser emission surface with a convex lens form. In JP-A-2000-76682, owing to the use of the VCSEL having a convex lens surface on a semiconductor accumulation body, the divergence angle of the laser beam can be diminished even in the case where the laser output is increased upon making the laser beam incident on a plastic optical fiber having a large propagation loss.
JP-A-2004-63707 proposes such a VCSEL that an aperture diameter d1 corresponding to the aperture 9 of the upper electrode 8 and an aperture diameter d2 defining the current confinement structure (oxide aperture) 12, as shown in FIG. 26, are in a particular range. The use of the structure blocks a higher mode of laser light having a strong electromagnetic field distribution in the circumference of the emission spot among the modes of laser light generated in the current confinement structure, so as to diminish the divergence angle, and thereby the coupling efficiency with an optical fiber is improved.
JP-A-10-56233 proposes such a VCSEL that contains an optical cavity forming optical loss and a loss determination device connected to the optical cavity. The loss determination device gradually increases optical loss of the optical cavity along with increase of the lateral distance from an optical axis, whereby the laser diode is suppressed from transferring to a multi-mode even when the laser current is increased, so as to enable high output power in a singe mode.
One of the characteristic features of the VCSEL as compared to an edge emitting laser diode is that the mode in the thickness direction, i.e., the vertical mode, exerts single mode oscillation, whereas the transverse mode is liable to exert a multi-mode oscillation, owing to the small volume of the active layer. It has been known with respect to the transverse mode that in the case where a particular mode is selectively removed, the fluctuation of optical output among the modes becomes unstable to increase the noise upon transmitting data, which is referred to as the mode partition noise, as described in “IEEE Journal of Quantum Electronics”, vol. 38, No. 8, pp. 1089-1096.
As an example of proposals from the standpoint of transversal mode control, U.S. Pat. No. 6,727,520 proposes such a VCSEL including a resonance reflector that contains a first patterned material layer having a thickness of an odd multiple of a quarter of the wavelength, a second material layer having a thickness of an odd multiple of a quarter of the wavelength, and a third material layer positioned immediately adjacent to the first material layer and having a refractive index that is greater than that of the first material layer. In this proposal, as shown in FIG. 27, the first material layer 300 is formed to have a curved surface by reflowing, and then the second material layer 302 and, depending on necessity, a reflection filter 304 are coated. The first material layer 300 constitutes a mode control resonance reflector having a convex shape at the top of the reflector, and the refractive index thereof is gradually increased from the edge toward the optical axis. The mode control is enabled by the gradual change in refractive index.
However, the conventional vertical cavity surface emitting laser diodes have the following problems. The lens surface of the VCSEL disclosed in JP-A-2000-76682 basically has a structure with upward convexity, i.e., the curvature radius on the output aperture side is smaller than the curvature radius on the active region side. In the case where the lens surface has upward convexity, it is difficult in view of the production process that the interlayer dielectric film and the upper electrode metallic film are produced with uniform thickness and sufficient adhesion strength. Accordingly, it is difficult to obtain such a VCSEL that has patterns of the interlayer dielectric film and the upper electrode material formed with excellent reproducibility. The proposal by U.S. Pat. No. 6,727,520 also has the same problems.
The proposals in JP-A-2000-76682 and U.S. Pat. No. 6,727,520 employ reflowing for forming the upward convex surface, but the patterns formed by the aforementioned measures basically suffer large in-plane fluctuation to cause large in-plane fluctuation in divergence angle. Therefore, the structure cannot be excellent in production stability of an array of a VCSEL, which is demanded to have uniformity in coupling efficiency.
The mode partition noise described in “IEEE Journal of Quantum Electronics”, vol. 38, No. 8, pp. 1089-1096 causes disorder in large signal characteristics, particularly eye patterns, and it has been recommended in multi-mode data communication that all the emitted oscillation modes are output. Output of all the oscillation modes from a laser can be attained by such a structure that the aperture diameter d1 defined by the upper electrode is equivalent to or larger than the aperture diameter d2 defined by the current confinement structure, but there is a problem that the divergence angle is increased, and the coupling efficiency with an optical fiber is deteriorated.
It is the current situation that there is no such VCSEL proposed that is excellent in reproducibility and production stability, has a small divergence angle, is excellent in small signal characteristics, and has small mode partition noise.